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The odd- ungulates are browsing and grazing mammals such as horses, tapirs and rhinoceroses whose hooves each feature an odd number of toes. The middle toe on each hoof is also usually larger than its neighbors. Together, odd-toed ungulates form the order Perissodactyla.

They are relatively large and, unlike the ruminant even-toed ungulates (artiodactyls), they have relatively simple stomachs. This is because they are hindgut fermenters, digesting plant cellulose in their intestines rather than in one or more stomachs.

Evolution

Homogal ax tapirinus skull.

Although no certain records are known prior to the early Eocene, the odd-toed ungulates probably arose in what is now Asia during the late Paleocene, less than 10 million years after the Cretaceous-Tertiary extinction event. By the start of the Eocene (55 million years ago) they had diversified and spread out to occupy several continents. The horses and tapirs both evolved in North America;^[2] the rhinoceroses appear to have developed in Asia from tapir-like animals and then recolonised the Americas during the middle Eocene (about 45 million years ago). There were approximately 15 families, of which only three survive (McKenna and Bell, 1997; Hooker, 2005). These families were very diverse in form and size; they included the enormous brontotheres and the bizarre chalicotheres. The largest perissodactyl, an Asian rhinoceros called Paraceratherium, reached 12 short tons (11 t), more than twice the weight of an elephant.

Perissodactyls were the dominant group of large terrestrial browsers right through the Oligocene. However, the rise of grasses in the Miocene (about 20 million years ago) saw a major change: the even-toed ungulates with their more complex stomachs were better able to adapt to a coarse, low-nutrition diet, and soon rose to prominence. Nevertheless, many odd-toed species survived and prospered until the late Pleistocene (about 10,000 years ago) when they faced the pressure of human hunting and habitat change.

Taxonomy

The members of the order fall into two suborders:

• Hippomorpha are odd-toed ungulates that are, today, fast runners with long legs and have only one toe. The only extant family of this subord er is Equidae (whose sole surviving genus is Equus), comprising the horse, zebra, donkey, onager, and allied species. The extinct, rhinoceros-like brontotheres are also included in this suborder. Both families probably descended from palaeotheres.
• Ceratomorpha have several functional toes; they are heavier than and move slower than the Hippomorpha. This suborder has two extant families: Tapiridae (tapirs) and Rhinocerotidae (rhinoceroses). The extinct chalicotheres may belong to this suborder as well.

The three surviving families of odd-toed ungulate are classified as follows.

• ORDER PERISSODACTYLA
  • Suborder Hippomorpha
    • Family Equidae: horses and allies, 10 species in one genus
      • Wild horse, Equus ferus
        • Przewalski's Horse, Equus ferus przewalskii
        • Domestic Horse, Equus ferus caballus
      • African Wild Ass Equus africanus
      • Onager or Asiatic Ass, Equus hemionus
      • Kiang or Tibetan Wild Ass, Equus kiang
      • Plains Zebra, Equus quagga
      • Mountain Zebra, Equus zebra
        • Cape Mountain Zebra, Equus zebra zebra
        • Hartmann's Mountain Zebra, Equus zebra hartmannae
      • Grevy's Zebra, Equus grevyi
  • Suborder Ceratomorpha
    • Family Tapiridae: tapirs, 5 species in one genus
      • Brazilian Tapir, Tapirus terrestris
      • Mountain Tapir, Tapirus pinchaque
      • Baird's Tapir, Tapirus bairdii
      • Malayan Tapir, Tapirus indicus
      • Black Dwarf Lowland Tapir, Tapirus pygmaeus (validity questionable)
    • Family Rhinocerotidae: rhinoceroses, 5 species in 4 genera
      • Black Rhinoceros, Diceros bicornis
      • White Rhinoceros, Ceratotherium simum
      • Indian Rhinoceros, Rhinoceros unicornis
      • Javan Rhinoceros, Rhinoceros sondaicus
      • Sumatran Rhinoceros, Dicerorhinus sumatrensis

It was thought that odd-toed ungulates are closely related to even-toed ungulates. But recent molecular comparisons show that even-toed and odd-toed ungulates may not form a clade. Instead, perissodactyls may be more closely related to carnivorans, bats and pangolins (and, by default, the Creodonts and Cimolestids). So, some biologists group the orders Perissodactyla, Carnivora, Chiroptera, Pholidota, Creodonta, and Cimolesta as the clade Pegasoferae (Nishihara et al., 2006; see Matthee et al., 2007 and Springer et al., 2007 for alternate views).

Characteristics

The tapir hooves

The living perissodactyls are a diverse group, with no generalized appearance. At one extreme are the lithe and graceful horses; on another, the huge, tank-like rhinoceroses; and in the middle, the vaguely pig-like tapirs. All extant perissodactyls are large, from the 180-kg Mountain Tapir to the 2,273-kg White Rhinoceros.

Extinct perissodactyls possessed a far more diverse range of forms, too, including the tiny, vaguely tapir-like paleotheres, the monstrous brontotheres, the knuckle-walking chalicotheres, and the gigantic rhin oceros Indricotherium, which dwarfed even elephants.

However, all perissodactyls, extinct and extant, have a mesaxonic foot structure. In other words, the symmetry of the foot passes through the third digit. This means that this digit holds the animal's weight. In equines, the mesaxonic foot has been modified so that the non-weight bearing digits have atrophied away, while the third toe has enlarged, so that modern equines have only one toe. Also, all perissodactyls are hindgut fermenters. Hindgut fermenters, in contrast to the ruminants, store digested food which has left the stomach in a pouch-like extension of the large intestine called the caecum (literally "cave"), where the food is digested by bacteria.

Social Structures

Today, the equines are the only social perissodactyls still extant. Horses organize themselves into small bands with a dominant mare at the top of the pecking order, as well as a resident stallion. Several bands will share a common territory, with some members of one band joining another band, every so often. These bands, in turn, form a "herd." Huge fossil beds made of the bones of hundreds or thousands of individuals suggest that many of the larger brontothere species, and some prehistoric rhinoceroses, such as Diceratherium were also social animals which organized themselves into herds. Modern-day rhinoceroses, on the other hand, are solitary animals which maintain territories, often attacking members of their own species when their space has been invaded. Tapirs, too, are solitary animals, though they are shy, retiring creatures which do not defend or maintain territori es.

Mating and Reproduction

A tapir calf

As with the males of many other animal groups, male perissodactyls often spar with each other for the privilege to mate with receptive females. A male which has found a female will attempt to taste her urine to see if she is in estrus. The female may also signal that she is in estrus, such as the whistling of cow Indian rhinoceroses and tapirs. Perissodactyls tend to have one foal or calf at a time. Very rarely, the female may have twins. Gestation is very long, from about 11 months in horses to 16 months for rhinoceroses. The calf or foal is capable of stand ing within moments of birth, but is very dependent on its mother. The young stays with its mother even after weaned, usually until it is chased off by the mother upon the birth of a new foal or calf. At this time, in horses, the foal will enter into the herd proper, later, young stallions are often chased off and join bachelor herds. With rhinos and tapirs, the newly weaned calf wanders away to search for new feeding grounds.

Humans and Conservation

Domestication

Humans have a historically long interaction with perissodactyls. The wild ass was the first equid to be domesticated,around 5000 BC in Egypt. Horses were domesticated 1000 years later. The zebroid, that is, a zebra hybrid, began appearing in zoos and menageries during the 19th century. During the 16th century, the Spaniards brought horses with them, and inadvertently rei ntroduced horses back into North America. While no rhinoceros has been domesticated, they have been captured for zoos and menageries since ancient times.

Conservation

The Przewalski's horse, one of the most endangered equids

The odd-toed ungulates have been among the most important herbivorous mammals, at times, they have been the dominant herbivores in many ecosystems. However, over the course of millions of years, many species went extinct due to climatic change, newer, coarser-leaved plants, predators, disease, and competition from other herbivores, particularly the artiodactyls. The Chalicotheriidae was the most recent family of perissodactyl to become entirely extinct. The perissodactyls' decline continues even today. Most species are listed as threatened species, and although no species are confirmed to be extinct, some subspecies have gone extinct. The quagga was hunted for its meat, the tarpan were hunted for sport, and a subspecies of Black Rhinoceros was hunted for its horn (as with all other African rhinoceros species).

Perissodactyls tend to do well in captivity, and there are many breeding programs in place to help replenish wild populations. The Przewalski's horse has been recently released back to the wild. Some of the captive breeding programs for some equids are unusual, in that breeders have been carefully selecting specimens to recreate various recently extinct equids, such as the Tarpan and Quagga. Most wild rhinoceroses are monitored, and some have their horns trimmed off to discourage horn-poachers. Even so, if conservations do not improve, it may very well be that the only living perissodactyls left will be the domesticated horse and donkey.

Two recently extinct equids

The Order Perissodactyla is a member of the Mirorder Altungulata. Here is the complete "parentage" of Perissodactyla:

• Domain: Eukaryota Whittaker & Margulis,1978 - eukaryotes
  • Kingdom: Animalia Linnaeus, 1758 - animals
    • Subkingdom: Bilateria (Hatschek, 1888) Cavalier-Smith, 1983 - bilaterians
      • Branch: Deuterostomia Grobben, 1908 - Deuterostomes
        • Infrakingdom: Chordonia (Haeckel, 1874) Cavalier-Smith, 1998
          • Phylum: Chordata Bateson, 1885 - Chordates
            • Subphylum: Vertebrata Cuvier, 1812 - Vertebrates
              • Infraphylum: Gnathostomata Auct. - Jawed Vertebrates
                • Superclass: Tetrapoda Goodrich, 1930 - Tetrapods
                  • Class: Mammalia C. Linnaeus, 1758 - Mammals
                    • Subclass: Theriiformes (Rowe, 1988) M.c. Mckenna & S.k. Bell, 1997
                      • Infraclass: Holotheria (Wible Et Al., 1995) M.c. Mckenna & S.k. Bell, 1997
                        • Superlegion: Trechnotheria Mckenna, 1975
                          • Legion: Cladotheria Mckenna, 1975
                            • Sublegion: Zatheria Mckenna, 1975
                              • Infralegion: Tribosphenida (Mckenna, 1975) M.c. Mckenna & S.k. Bell, 1997
                                • Supercohort: Theria (Parker & Haswell, 1897) M.c. Mckenna & S.k. Bell, 1997 - a genus of Click Beetles (Elateridae)
                                  • Cohort: Placentalia (Owen, 1837) M.c. Mckenna & S.k. Bell, 1997 - Placentals
                                    • Magnorder: Epitheria (Mckenna, 1975) M.c. Mckenna & S.k. Bell, 1997
                                      • Superorder: Preptotheria (Mckenna, 1975) Mckenna, in Stucky & Mckenna, in Benton, Ed., 1993
                                        • Grandorder: Ungulata (C. Linnaeus, 1766) Mckenna, 1975
                                          • Mirorder: Altungulata (Prothero & Schoch, 1989) M.c. Mckenna & S.k. Bell, 1997
                                            • Order: Perissodactyla Owen, 1848 - Odd-Toed Ungulates